Function icon_preview 

fn icon_preview(
    data: &Drawing<'_>,
    shape: ShapeId,
    bbox: Rect,
    preview: Rect,
) -> Option<(ShapeId, Vec2)>

The icon a resized block carries, and the offset to preview it at: exactly where the drop puts it (see icon_rect_after_resize), so the icon tracks the block through the drag instead of jumping into place on release.